The present invention relates to a method of recording images and an apparatus therefor, such as printers, facsimiles, and copying machines; and, more particularly, the invention relates to a method of recording multi-color images and an apparatus therefor.
In general, a multi-color image recording apparatus records multi-color images by combining a plurality of images of different colors. It has been well-known that full-color recording can be accomplished by combining three primary colors (cyan, magenta, and yellow). Most of the color image recording apparatuses provide a black color in addition these three primary colors to record full-color images. Although a black color can be made by mixing the three primary colors, a black color is added because it is frequently used (e.g. for characters) and plays a significant role in image formation.
For high-quality image recording by an image recording apparatus using three primary colors, it is required to combine the images of three primary colors precisely. Particularly when three primary colors are mixed to form fine characters and lines, a little deviation of the primary colors will degrade the quality of the images. To prevent this, a black color has been added and is used individually. It is needless to say that the deviation of colors must also be avoided in recording of multi-color images because such a deviation has a great influence on color reproduction and image quality.
A laser printer using an electrophotographic process has been widely used, since it is capable of fast and high-quality recording on various recording media. However, in color image recording in the electrophotographic process is hard to combine independent images of primary colors precisely, although a high-precision combination of color images is required.
It is very hard to achieve, with a high precision, a combination of color images particularly in a tandem recording system which is adapted to fast color image recording and uses a plurality of completely-independent color printing systems. Therefore, various means have been proposed to increase the color combining precision (the color overlapping accuracy).
For example, the publication “Four-Drum Digital Color El Electrophotographic System” (pp. 101-104, “Japan Hardcopy” '91, 1991) has been reported a method which provides two rotary polygon mirrors on a single motor shaft and causes a total of laser beams (two laser beams on each side) to scan at an identical speed in a four-color printing apparatus. This method has an effect to eliminate scanning errors of color images.
Japanese application patent laid-open publication No. Hei 7-160085 (1995) and Japanese application patent laid-open publication No. Hei 4-147280 (1992) have disclosed other methods involving the detecting of recording offsets of color images by positional sensors or the like and controlling the angles of mirrors in the laser optical systems and controlling a plurality of LED arrays.
Further, Japanese application patent laid-open publication No. Hei 7-168414 (1995) has disclosed a method of adjusting the parallelism of laser exposing units and exposure timing by providing detection windows on both sides of each laser exposing window, controlling the movement of each mirror so that each laser beam may scan along a preset scanning line and controlling the exposure start signal to match the exposure start timing of each exposure unit with a preset value. This method can control the position and timing of laser beams of the exposure units.
It is very hard to completely correct color offsets of color images even by the above-stated methods. This is partly because recent high-quality printing units have higher resolutions.
The typical resolution in color image recording is 600 dots per inch (dpi) or higher. At 600 dpi, pixels are recorded at a pitch of about 42 micrometers. It is mechanically hard to keep a steady pitch of 100 micrometers or less and, consequently, high-quality recording apparatuses becomes very expensive. This is also true of the above-stated recording apparatus having two rotary polygon mirrors on a single motor shaft.
Similarly, the control means and mechanisms which have been disclosed in Japanese application patent laid-open publication No. Hei 7-160085 (1995), Japanese application patent laid-open publication No. Hei 4-147280 (1992), and Japanese application patent laid-open publication No. Hei 7-168414 (1995) cannot avoid being complicated and expensive.
Further, a method disclosed in Japanese application patent laid-open publication No. Hei 7-168414 (1995), which precisely controls the position and exposure timing of each laser beam, is not able to improve the precision of combining color images into a multi-color image because the precision of combining color images in a tandem system is affected not only by deviations in laser beam positions and timing of exposure units, but also parallelism of the whole color printing systems and rotational speed errors of a photoconductive means.
As explained above, it is hard to say that the conventional methods can assure a satisfactory color combining precision; and, further, the conventional methods cannot be free from making the apparatuses complicated and expensive. Further, the conventional methods are apt to be affected by the environment and time lapses and cannot assure an optimum color positioning precision.
Furthermore, the world will want higher resolutions in a color recording apparatus which cannot be accomplished by conventional mechanical precision or precision control methods.